The invention relates to an actuator unit for an electronically controlled hydraulic vehicle brake system, with a master brake cylinder, which can be actuated by a brake pedal, and a reservoir for hydraulic fluid.
This actuator unit is a so-called electrohydraulic brake unit, in which the driver""s braking requirement, which is passed onto the brake pedal, is only delivered to a device for simulating the brake pedal behaviour in normal operation, while the actual action on the vehicle brakes by pressurised hydraulic fluid is effected via an electronically activated pump, which builds up, maintains and/or reduces the hydraulic pressure in the wheel brakes.
The hydraulic fluid which is delivered by the pump is determined by an activation signal which is generated by a sensor which is coupled either directly or indirectly to the brake pedal.
In emergency operation the hydraulic fluid displaced by the master brake cylinder, which can be actuated by the brake pedal, is fed directly into the vehicle brakes via a connection which is then established. A vehicle brake system of this kind is also called a xe2x80x9cpush-throughxe2x80x9d-capable xe2x80x9cbrake-by-wirexe2x80x9d vehicle brake system.
The brake system described in DE 31 24 755 A1 comprises a first brake circuit section and a second brake circuit section. The first brake circuit section comprises a master cylinder, a displacement simulator for simulating the brake behaviour and a first pressure measuring device. The second brake circuit section comprises a wheel brake cylinder and a second pressure measuring device. The two brake circuit sections can be connected and separated by one or two multiposition valves.
The multiposition valve is used both for the brake boost function and for anti-locking.
When the device is intact the master cylinder pressure only serves as control pressure, while the pressure at the wheel cylinder is fed in as servo-pressure. Should the servo-pressure supply fail, the first and the second brake circuit sections are connected by the multiposition valve, so that brake fluid flows into the wheel brake cylinder when the brake pedal is actuated. At this moment the brake-by-wire brake system function is no longer operational.
DE 196 38 102 C1 discloses a vehicle brake system in which to an actuating unit for an electronically controlled brake system comprises a master brake cylinder which can be actuated by a brake pedal. Two brake circuits extend from the master brake cylinder in order to supply wheel brakes with hydraulic fluid from a reservoir. A first valve device is disposed between the master brake cylinder and each brake circuit of the wheel brakes to open the connection between the master brake cylinder and each brake circuit of the wheel brakes with spring assistance (basic position) and block it by electromagnetic actuation (actuating position).
If the brake system is faultless, the first valve device takes up its actuating position, so that the supply of hydraulic fluid to the wheel brake is effected by an electronically controlled pressure source. If, on the other hand, a fault occurs in the electronically controlled system, the first valve arrangement returns to its basic position, so that emergency actuation of the wheel brake can be carried out from the master brake cylinder.
The outlet of the master brake cylinder is connected to a simulation device parallel to the first valve arrangement and the wheel brake. When the first valve device is in its actuating position, i.e. the wheel brake is supplied with hydraulic fluid (solely) through the electronically controlled pressure source, the simulation device serves to impart to the driver a pedal characteristic which is usual in a conventional brake system when the brake pedal is actuated.
A second valve device is disposed in the connection between the outlet of the master brake cylinder and the simulation device to block the connection between the master brake cylinder and the simulation device with spring assistance and open it by electromagnetic actuation. It is thereby possible to block the connection from the master brake cylinder to the simulation device during emergency braking of the wheel brake, so that no hydraulic fluid volume can escape into the simulation device during emergency actuation of the wheel brake. The second valve arrangement then changes over to its basic position when the first valve device also switches (or has switched) into its basic position, which means that a fault (for example undervoltage) has occurred in the electronically controlled system.
This known arrangement requires either a combined valve arrangement consisting of the first and the second valve device, or two first and second valve devices to be activated separately. The overall design of this arrangement is highly complicated. There is a permanent requirement for simplification, with unchangingxe2x80x94if not even increasingxe2x80x94quality and safety, especially on account of the competitive situation and the high pressure on costs to which motor vehicle manufacturers subject their suppliers.
An object of the invention is therefore to provide a vehicle brake system of the above-mentioned type which is of a simpler design with the same power features.
According to the invention, an actuator unit for an electronically controlled hydraulic vehicle brake system is provided which has a master brake cylinder in which a first piston, which can be displaced by an actuator, is disposed, a first brake circuit which can be connected by a valve arrangement in a basic position to the master brake cylinder or separated from the latter thereby in an actuating position, and a device which can be connected to a hydraulic chamber of the master brake cylinder to simulate the brake pedal behaviour, wherein a second piston is disposed in a displaceable manner in the master brake cylinder and the displacement of one piston causes the displacement of the other piston, wherein the device for simulating the brake pedal behaviour is switched to an active or inactive mode through the displacement.
This arrangement has the advantage of not requiring the electrically actuated valve device between the master brake cylinder and the simulation unit. As the known valve device is in its actuating position during normal operation of the vehicle brake system for safety reasons, this results in a permanent current consumption with corresponding heating. The invention also simplifies the design, which is usually structurally complicated, of the vehicle brake system, as it does not include a valve device as a separate part. Moreover, the invention teaches a procedure which does not involve electrical activation of an electromagnetic valve arrangement of this kind.
In a preferred embodiment the two pistons define with one another and with the wall of the master brake cylinder a respective first and second hydraulic chamber, wherein hydraulic fluid is held in these hydraulic chambers.
The second piston also comprises a duct leading from one of the hydraulic chambers to a lateral outlet, wherein the fluid connection leading to the device for simulating the brake pedal behaviour is disposed such that, depending on the position of the second piston, it is aligned with the outlet of the duct when the brake circuit is separated from the master brake cylinder by the first valve arrangement.
Here the device for simulating the brake pedal behaviour has a spring arrangement which is set up to act against the hydraulic pressure from the master brake cylinder. A characteristic modelling device is also associated with this device in terms of action.
In a first embodiment of the invention the characteristic modelling device is disposed in the connection between the master brake cylinder and the device for simulating the brake pedal behaviour.
In a second embodiment of the invention, which is substantially of a more compact construction, the characteristic modelling device is disposed in one of the pistons in the master brake cylinder, preferably in the flow path between one of the hydraulic chambers and the lateral outlet of the second piston.
Finally, the characteristic modelling device may also be disposed in the connection between the device for simulating the brake pedal behaviour and a hydraulic reservoir.
The device for simulating the brake pedal behaviour is formed by a cylinder/piston arrangement, in which a piston is loaded by a spring arrangement.
No hydraulic fluid can flow from the master brake cylinder into the (first) brake circuit when the arrangement is in a normal operating mode (active) with valve arrangement between the master brake cylinder and the brake circuit in the (closed) actuating position. If the first piston is actuated in this situation, the piston (as a rule the second) which switches the device for simulating the brake pedal behaviour to active or inactive mode is displaced into a position in which this device is switched to active mode. As soon as the switch to active mode has taken place, the hydraulic fluid in the first hydraulic chamber is forced through the duct in the second piston into the device for simulating the brake pedal behaviour.
Hydraulic fluid can flow into the (first) brake circuit when the arrangement is in an emergency operating mode (passive) with valve arrangement between the master brake cylinder and the brake circuit in the (open) basic position. If the first piston is actuated in this situation, the piston (as a rule the second) which switches the device for simulating the brake pedal behaviour to active or inactive mode is displaced by a spring arrangement or similar into a position in which this device is switched to inactive mode.
The first piston, when actuated, therefore forces the hydraulic fluid in the first hydraulic chamber into the first brake circuit, and the first piston simultaneously displaces the second piston, so that the second hydraulic chamber is also diminished and hydraulic fluid in the latter is forced into the second brake circuit.